1. Field of the Invention
The present invention relates to an uninterruptive switching regulator which has a high power factor and is reduced in a higher harmonic component in a primary-side a.c. input current as well as an output ripple, and more particularly to a technique for realizing an uninterruptive switching regulator of the serf-excitation type which is reduced in the number of parts, downsized and inexpensive.
2. Description of the Related Art
In recent years, as computer equipments have been spread, attention has been given to an influence of a momentary power failure on a commercial power supply. A variety of uninterruptive d.c. regulated power supplies have been proposed, one of which has been disclosed in Japanese Patent Unexamined Publication No. Hei 6-205546 which has already been proposed by the present inventor. This includes a basic circuit shown in FIG. 6, which is comprised of: (1) a rectifier circuit 102 having no smoothing circuit for rectifying an alternating current from an a.c. power supply 100; (2) a primary side circuit 110 in which a primary winding 106 of a high-frequency transformer 104 and a primary side switching element 108 are connected in series to an output side of the rectifier circuit 102, to develop a primary side high frequency pulse voltage through the primary side switching element 108; (3) a secondary side circuit 116 in which a rectifying and smoothing circuit is connected to a secondary winding 112 of the above high-frequency transformer 104, to supply a d.c. output voltage to a load 114; (4) a charge circuit 126 in which an electrical double layer capacitor 120, a choke coil 122 and a high-frequency rectifier diode 124 are connected in series to a tertiary winding 118 of the above high-frequency transformer 104; (5) a discharge circuit 132 in which the above electrical double layer capacitor 120 and a tertiary side switching element 130 are connected in series between a winding start portion of the above tertiary winding 118 and a winding end portion thereof or a tap 128 drawn out from the middle portion of that winding; and (6) a pulse width modulation control switching control circuit 134 which outputs a pulse signal in synchronism with the above primary side switching element 108 and the tertiary side switching element 130 to perform a switching operation and also modulates the pulse width of a pulse signal in response to the fluctuation of an output voltage of the above secondary side circuit 116 to control the pulse width of the primary side high-frequency pulse voltage.
The conventional uninterruptive switching regulator like this operates as follows: When the output voltage of the rectifier circuit 102 exceeds a predetermined level, a sine-wave alternating current supplied from the a.c. power supply 100 is supplied to the primary side of the high-frequency transformer 104 in the form of a full-wave sine-wave pulsating current waveform. Then, the full-wave sine-wave pulsating current flows in the primary side switching element 108 through the primary winding 106 and a reverse current preventing diode 136, and is chopped while being controlled by the switching control circuit 134, then being outputted to the secondary side circuit 116.
Also, when the output voltage of the rectifier circuit 102 exceeds a predetermined level, a current .sub.1, which flows in the primary side circuit 110 when the primary side switching element 108 is on, causes a current i.sub.2 shown in the figure to flow in a tertiary side circuit 138. The current i.sub.2 is smoothed by the choke coil 122 and charges the electrical double layer capacitor 120.
On the other hand, when the output voltage of the rectifier circuit 102 does not exceed the predetermined level, the charged voltage of the electrical double layer capacitor 120 becomes higher than an induced voltage of the tertiary winding 118 which is caused by the current i.sub.1, with the result that a discharge current i.sub.3 flows through a winding portion between the winding start portion of the tertiary winding 118 and the halfway tap 128. The discharge current i.sub.3 is chopped by the tertiary side switching element 130 and outputted to the secondary side circuit 116.
With such operation, a given secondary-side output can be obtained even when the momentary power failure, etc., make the output voltage of the rectifier circuit 102 low, resulting in an uninterruptive switching regulator.
However, the conventional uninterruptive switching regulator suffers from such a problem that its separate excitation type makes it difficult to downsize the device and to lower the costs. In other words, in the above-mentioned conventional uninterruptive switching regulator, the pulse width modulation control circuit (PWM) 134 is used for the purpose of obtaining the primary side pulse voltage, which is a basic part separately controlled and separately attached. The requirement of such a basic part which is separately controlled and separately attached causes not only an increase in the number of parts but also an increase in the costs, which is not preferred.